SELECTED QUARTERLY CONSOLIDATED FINANCIAL DATA - Selected quarterly consolidated financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Income
Interest and dividend income									$ 35,521	$ 34,454	$ 34,610
Interest expense									4,238	3,849	3,026
Net interest and dividend income									31,283	30,605	31,584
Provision (benefit) for loan losses									1,020	1,835	917
Gains on securities, net									178	584	192
Trust and Wealth Advisory									3,477	3,338	3,265
Service charges and fees									3,718	3,133	2,986
Gains on sales of mortgage loans, net									125	229	274
Mortgage servicing, net									255	156	47
Other									483	451	510
Non-interest income									8,236	7,891	7,274
Non-interest expense									29,329	27,387	25,921
Income before income taxes									9,170	9,274	12,020
Net income allocated to common stock									6,201	6,633	$ 8,227
Financial Condition
Total assets	$ 986,984				$ 935,366				$ 986,984	$ 935,366
Per Common Share Data
Earnings, basic									$ 2.25	$ 2.43	$ 3.04
Earnings, diluted									2.24	2.41	3.02
Cash dividends declared									$ 1.12	$ 1.12	$ 1.12
Statistical Data
Weighted average common shares outstanding, basic									2,755	2,733	2,706
Selected quarterly consolidated financial data
Statement of Income
Interest and dividend income	9,167	$ 8,882	$ 8,687	$ 8,785	8,679	$ 8,670	$ 8,522	$ 8,583
Interest expense	1,143	1,116	1,026	953	992	983	955	919
Net interest and dividend income	8,024	7,766	7,661	7,832	7,687	7,687	7,567	7,664
Provision (benefit) for loan losses	67	237	364	352	503	344	525	463
Gains on securities, net	192		(14)		427	9	146	2
Trust and Wealth Advisory	857	874	892	854	821	849	884	784
Service charges and fees	919	935	902	962	856	822	753	702
Gains on sales of mortgage loans, net	21	25	30	49	78	55	57	39
Mortgage servicing, net	75	104	31	45	37	40	45	34
Other	118	142	110	113	108	113	116	114
Non-interest income	2,182	2,080	1,951	2,023	2,327	1,888	2,001	1,675
Non-interest expense	8,052	7,220	6,751	7,306	7,411	6,500	6,640	6,837
Income before income taxes	2,087	2,389	2,497	2,197	2,100	2,731	2,403	2,039
Income tax provision	1,011	695	615	593	580	812	669	527
Net income	1,076	1,694	1,882	1,604	1,520	1,919	1,734	1,512
Net income allocated to common stock	1,064	1,677	1,866	1,594	1,509	1,904	1,721	1,499
Financial Condition
Total assets	986,984	979,469	974,806	939,549	935,366	928,445	913,494	891,804	$ 986,984	$ 935,366
Loans, net	801,703	784,136	771,850	764,665	763,184	753,623	749,523	728,845	801,703	763,184
Allowance for loan losses	6,776	6,494	6,493	6,285	6,127	5,892	5,718	5,877	6,776	6,127
Securities	82,860	88,546	84,468	80,359	82,834	79,738	83,874	82,151	82,860	82,834
Deposits	815,495	831,989	811,341	772,416	781,770	786,730	754,471	755,658	815,495	781,770
Repurchase agreements	1,668	4,529	2,126	2,350	5,535	3,581	3,355	2,620	1,668	5,535
FHLBB advances	54,422	27,364	47,302	52,745	37,188	27,134	47,083	27,031	54,422	37,188
Shareholders' equity	97,514	97,526	96,545	95,221	94,007	93,554	92,584	91,402	97,514	94,007
Non-performing assets	$ 7,354	$ 12,257	$ 11,690	$ 10,890	$ 12,565	$ 14,496	$ 14,579	$ 16,829	$ 7,354	$ 12,565
Per Common Share Data
Earnings, basic	$ 0.39	$ 0.61	$ 0.68	$ 0.58	$ 0.55	$ 0.70	$ 0.63	$ 0.55
Earnings, diluted	0.38	0.60	0.67	0.58	0.55	0.69	0.63	0.55
Cash dividends declared	0.28	0.28	0.28	0.28	0.28	0.28	0.28	0.28
Cash dividends paid	0.28	0.28	0.28	0.28	0.28	0.28	0.28	0.28
Book value	35.01	35.01	34.66	34.38	28.90	28.63	28.28	27.84
Market price: High	48.35	46.90	41.50	40.00	38.15	32.52	32.75	34.43
Market price: Low	$ 44.00	$ 40.40	$ 38.50	$ 37.30	$ 30.92	$ 29.60	$ 29.78	$ 29.51
Statistical Data
Net interest margin (fully tax equivalent)	3.58%	3.50%	3.58%	3.69%	3.45%	3.57%	3.71%	3.79%
Efficiency ratio (fully tax equivalent)	6490.00%	6718.00%	6656.00%	6868.00%	6708.00%	6413.00%	6651.00%	6928.00%
Return on average assets	43.00%	69.00%	77.00%	70.00%	65.00%	81.00%	77.00%	68.00%
Return on average shareholders' equity	438.00%	689.00%	782.00%	683.00%	643.00%	820.00%	758.00%	668.00%
Weighted average common shares outstanding, basic	2,757	2,757	2,757	2,749	2,737	2,737	2,735	2,723
Weighted average common shares outstanding, diluted	2,778	2,777	2,775	2,768	2,755	2,751	2,749	2,741